Electric machines may be used in motor vehicles as so-called starter generators in order to, on the one hand, start the internal combustion engine during motor operation of the electric machine and, on the other hand, to generate power for the vehicle electrical system and for charging the motor vehicle battery during generator operation of the electric machine. Starter generators may be connected to the internal combustion engine or the crankshaft via a belt drive.
In particular, separately excited three-phase synchronous machines are suitable for use as belt-driven starter generators (BSG) since their motor-based torque is particularly easy to control. A desirable torque may be set by appropriate control of the rotor winding (excitation coil) and/or the stator winding (e.g., three or five stator phases are common). A modulation of the torque over time may be preferred to achieve a preferably low-noise and low-vibration starting process.
To start the internal combustion engine, the drive torque of the starter generator is transmitted via the belt to the crankshaft of the internal combustion engine, in order to accelerate the crankshaft to a minimum rotational speed. Once the minimum rotational speed has been reached, a combustible mixture in the combustion chamber is ignited at a defined point in time.
To start the internal combustion engine, powerful starter generators must be used, which are large, heavy, and expensive. It is therefore desirable to save space, weight, and costs.